1. Field of Application
The present invention relates to a method of controlling a DC-to-DC converter.
2. Description of Related Art
Various types of isolated DC-to-DC converter are known, and widely utilized. For example, the DC-to-DC converter example shown in FIG. 1 is of such a type, having two switching elements Q1, Q2 at the input side and two switching elements Q3, Q4 at the output side, with each of the switching elements controlled for on/off switching in a predetermined sequence. Unless otherwise indicated, it will be assumed in the following that each switching element of a described DC-to-DC converter is a MOSFET, with switching controlled by altering the gate voltage.
FIG. 33 is a timing diagram showing an example of a prior art type of basic gate control sequence for controlling the switching elements Q1 to Q4 in the DC-to-DC converter of FIG. 1. As shown in FIG. 33, the switching elements Q1, Q3 and the switching elements Q2, Q4 operate in synchronism, with the switching elements Q1, Q2 performing complementary on/off switching, and the switching elements Q3, Q4 similarly performing complementary on/off switching. At each transition interval in which the switching elements Q1, Q2 change between the on/off (i.e., closed and open) states and each transition interval in which the switching elements Q3, Q4 change between the on/off states, respective dead times Td1, Td2 are provided. These are intervals in which both of the switching elements (i.e., both switching elements Q1, Q2, or both switching elements Q3, Q4) are in the OFF state.
Such a DC-to-DC converter can operate in a boost mode, for converting input electrical power supplied to the No. 1 voltage system circuit 100 to output power that is supplied to the No. 2 voltage system circuit 200 with the output voltage of the No. 2 voltage system circuit 200 being higher than the input voltage of the No. 1 voltage system circuit 100. In the boost mode, each time the input side switching element Q1 or switching element Q2 becomes turned off, the magnetic energy stored in the choke coil L causes a high level of voltage surge to be applied to the switching element that is switched off. If the DC-to-DC converter is operated in a step-down mode (i.e., back operation) in which electrical power is converted from being supplied to the No. 2 voltage system circuit 200 to being supplied to the No. 1 voltage system circuit 100, with the input voltage of the No. 2 voltage system circuit 200 being higher than the output voltage of the No. 1 voltage system circuit 100, and if the switching elements Q1, Q2 are respective MOSFETs that each have a parasitic diode, then due to the action of such parasitic diodes, the level of voltage surge that occurs at the time of switch-off of the switching element Q1 or switching element Q2 will generally be negligible or very low.
During operation in the step-down mode, some voltage surge occurs when a parasitic diode becomes reverse-biased. However this voltage surge is mainly due to resonance between the leakage inductance of the transformer and the stray capacitances of the switching elements, and is generally small.
However the levels of voltage surge generated due to switch-off of the switching elements Q1, Q2 during operation in the boost mode are substantial, and can apply an excessive amount of stress on these switching elements, as well as resulting in generation of electrical noise. Hence, these voltage surges present a serious problem.
With such a bidirectional DC-to-DC converter, which can be operated in the boost mode or in the back (step-down) mode, due to the high levels of voltage surge generated during boost operation it is necessary to use devices (generally MOSFETs) that have a high level of withstand voltage, as the switching elements Q1, Q2. For that reason, as described for example in Japanese patent first publication No. 2000-184710, it is known in the prior art to connect a snubber circuit to these switching elements, for absorbing the voltage surges.
Furthermore, with types of DC-to-DC converter of the form shown in FIGS. 14 and 24, which do not require the use of a choke coil, similar problems arise, resulting from the effects of the excitation inductance and leakage inductance of the transformer.
If a snubber circuit is added, then the number of components of the DC-to-DC converter is increased, and the circuit configuration made more complex, so that parts costs are increased.